While Unit Two retains abundant historical architectural elements without compromising any of today's technological features, the three other homes present contemporary living spaces.

Today it is rare to find exquisitely detailed hand-carved woodwork of a Gilded Age mansion combined with top-of-the-line 21st century craftsmanship in a spacious condominium, but that is exactly what exists in Unit Two at 305 Commonwealth Ave. in the Back Bay.

This distinctive duplex is one of four ultra-luxurious residences within the Thayer Mansion, designed and built between 1884 and 1886 by the eminent architectural firm of Peabody and Stearns. Hexagon Properties acquired the brick bowfront property about two-and-a-half years ago, and developer Sandra Edgerley assembled a team that includes architect Guy Grassi of Grassi Design Group, project manager Richard Feldman of Feldman Development Partners and general contractor Martins Construction Company to create these magnificent homes.

Tracy Campion of Campion and Company Real Estate is the exclusive broker.

Unit Two is a stunning 4,286-square-foot duplex with three-plus bedrooms and three full baths, plus a private deck and garage parking. It is listed at $10,990,000.

While Unit Two retains abundant historical architectural elements without compromising any of today’s technological features, the three other homes present contemporary living spaces.

The penthouse, also a duplex, encompasses 4,028 square feet and is nearing completion. It will have three-plus bedrooms, three full baths and two half baths, plus an expansive 920-square-foot tiered ipe deck with an outdoor kitchen, a gas fireplace and two garage parking spots.

Unit Three has two-plus bedrooms and three baths within its 2,261 square feet of living space. Listed at $5,900,000, this condo also has a garage parking space and exclusive use of an outdoor courtyard.

The smallest condo, which is on the garden level, comprises 552 square feet and has one bedroom and one bath. This could be purchased as an au pair suite or an in-law apartment for the owners of one of the upper floor residences. This unit could also be ideal for an individual looking to live simply, a “home away from home” for people working in the city who prefer not to have a daily commute, or as an “escape to the city” home for suburban/country dwellers who prefer not to stay in a hotel when visiting.

Wide brownstone steps, flanked by huge pillars, lead up to the arched recessed entry with intricate ornamentation. The vestibule features a high domed ceiling and a second door that continues into an arched lobby with shoulder-high paneling and a sumptuously carved staircase that ascends to the upper levels. A new elevator lies behind the stairs.

The entrance to Unit Two is on the left and opens into an elegant dining room dominated by a handsome fireplace with a marble surround and an enormously wide mantelpiece with intricate carvings; the overmantel has niches for sconces on either side of a Greek frieze.

The coffered ceiling rimmed by a complimentary frieze soars above the restored quarter-sawn oak flooring laid in a chevron pattern, and the room is surrounded by high, paneled wainscoting. Solid pocket doors separate the dining room from the living room, equally beautiful.

The four tall, sun-filled bow windows with leaded glass transoms look out to the Commonwealth Avenue Mall, and the wainscoting here is paneled and waist-high. The flooring is traditional. A candelabrum centers the ceiling, bordered with a deep 7-inch frieze, and matching sconces illuminate another fireplace with a wood mantelpiece, a marble surround and a mirrored overmantel.

On the other side of the dining room is a huge family room/kitchen, which is breathtaking. Measuring slightly more than 28-by-20 feet, this room features an enormous fireplace with a three-niche overmantel. The original marble surround is black with white swirls, and the hearth has a black and white diamond checkerboard.

The elaborate woodcarvings of the mantelpiece have been replicated with laser-cut pieces with a floral motif for the custom-designed range hood across the room, which is divided by a large quarter-sawn oak center island.

The counter comprises Nero Marquina stone, which looks like the original marble, and the matching backsplash above the Ilve Majestic black enamel dual fuel range has a pot filler.

Other high-end appliances include a Bosch dishwasher and a pair of Sub-Zero refrigerators with bottom freezers; all three have integrated paneling to match the abundant cabinetry. The black farmer’s sink is set into the center island.

At the far end of the room are three windows with leaded glass transoms, like those in the living room, and a wide banquette with closets on either side. The floor is quarter-sawn oak in an oversized parquet pattern, and the coffered ceiling boasts natural wood. It looks like honeycomb.

Beyond the kitchen is an ell that includes a second stairway, storage space, a full bath with a shower and a den.

The master bedroom suite on the upper level occupies nearly half of this level. Overlooking the mall, it boasts original hardwood flooring and a high honeycomb ceiling with a chandelier. Bookcases surround the room, interrupted by another fireplace, and in front of the bow windows is a wide window seat.

A study off the bedroom with even more embellished woodwork that rivals that in the family room offers a retreat and more storage.

The suite includes a spacious dressing room lined with open shelving and hanging rods, anchored with a marble-topped dressing table. Next to it is a lavish master bath that sparkles with Bianco Statuario marble on the walls and floor. The double vanity is topped with marble as well. A stand-alone 66-inch elliptical tub and a glass-enclosed steam shower offer a delightful bathing experience.

Two more bedrooms are on this level as well as a shared bath with a double vanity and glass-enclosed shower. One bedroom has a “twindle,” a built-in bed in front of the window. Opposite the bath is an elevator that gives direct access to the garage. The hallway continues to a large laundry with a side-by-side Electrolux washer and dryer and sink and a private Ipe deck.

An interesting feature of the Thayer Mansion is the garage turntable, which allows a 180-degree rotation for easy exiting. Unit Two has two tandem garage parking spaces.